A global positioning system is a technique of receiving navigation signals transmitted from satellites to calculate a current position of a receiver. As is generally known, in order to calculate the position of the receiver, the navigation signals need to be received from at least four satellites and thus as the number of satellites from which the navigation signals are transmitted increases, an accurate position of the receiver may be generally calculated.
However, in the case where the navigation signals are blocked by obstacles such as a mountainous terrain, buildings in urban areas, trees, and the like, the number of satellites from which the navigation signals are transmitted decreases and thus navigation performance is deteriorated and when the number of satellites from which the navigation signals are transmitted decreases to 3 or less, the position of the receiver may not be calculated.